Niobium sintered body for capacitor and replace with process for producing same

ABSTRACT

A niobium sintered body for a capacitor, which exhibits an LC value of not larger than 300 μA/g as measured after an electrolytic oxide film is formed thereon. The sintered body preferably exhibits a product (CV) [i.e., a product of capacity (C) with electrolysis voltage (V)] of at least 40,000 μF·V/g. The sintered body is produced by sintering a niobium powder containing at least one niobium compound selected from niobium nitride, niobium carbide and niobium boride. A capacitor manufactured from the sintered body has a large capacity per unit weight and good leak current characteristics. Especially, a sintered body made of a niobium powder having a large average degree of roundness has a relatively large porosity and a good packed density, and a capacitor manufactured from this sintered body has a large capacity and good withstand voltage characteristics.

This application claims benefit of No. 60/108986 filed Nov. 18, 1998.

TECHNICAL FIELD

This invention relates to a niobium sintered body for capacitor, whichis capable of giving a capacitor having an enhanced capacity per unitweight and exhibiting good leak current (hereinafter abbreviated to“LC”) characteristics. Further, it relates to the capacitor, and aprocess for producing the capacitor.

BACKGROUND ART

Capacitors used for electronic equipment such as portable telephones andpersonal computers are desired to be of a small volume. Tantalumelectrolytic capacitors are popularly used because they have a largecapacity relative to their size and good characteristics. As an anode ofthe tantalum electrolytic capacitors, a sintered body of a tantalumpowder is generally used. To further enhance the capacity of thetantalum electrolytic capacitor, it is necessary to increase the weightof sintered body, or to prepare a sintered body from an ultrafinetantalum powder having an enhanced specific surface area.

The increase of the weight of sintered body inevitably leads to anincrease of size of capacitor, and thus a capacitor having a desirablysmall volume cannot be obtained. When a sintered body is prepared froman ultrafine tantalum powder having an enhanced specific surface area,the tantalum sintered body has pores which have a reduced diameter andpart of which are clogged upon sintering, and therefore, the sinteredbody is difficult to impregnate with a cathode material at anafter-treating step. To solve these problems, a proposal has been madewherein a sintered body is made of a powdery material having adielectric constant larger than that of tantalum, such as niobium ortitanium.

However, a conventional capacitor using an electrode made of a sinteredbody of a powdery material having a large dielectric constant hasanother problem such that the LC characteristics are not satisfactoryand the capacitor is of poor practical use. More specifically, in thecase where a sintered body is made from a high-capacity tantalum powderexhibiting a product (CV), i.e., a product of capacity×electrolysisvoltage, of 40,000 μF·V/g, the LC value as measured on a sintered body,which has been subjected to electrolytic oxidation, at a voltage of 70%of the electrolysis voltage when three minutes elapsed from theelectrolytic oxidation, is usually approximately 30 μA/g. In contrast, asintered body made of a conventional niobium powder in a similar mannerhas an LC value more than 100 times larger than that of the tantalumpowder. Thus, a capacitor manufactured from a niobium sintered body isnot satisfactory in LC characteristics and leads to enhancement inelectric power consumption of an electrical equipment, and the capacitorhas poor reliability.

DISCLOSURE OF THE INVENTION

In view of the foregoing prior art, a primary object of the presentinvention is to provide a capacitor exhibiting good LC characteristicsand having an enhanced capacity per unit weight.

As results of an extensive research, the present inventors succeeded indevelopment of a niobium sintered body giving a capacitor having areduced LC value, and have completed this invention.

In a first aspect of the invention, there is provided a niobium sinteredbody for a capacitor, which is made of a niobium powder, andcharacterized by exhibiting an LC value of not larger than 300 μA/g asmeasured after an electrolytic oxide film is formed thereon.

The niobium sintered body for a capacitor preferably exhibits a product(CV) [i.e., a product of capacity (C) with electrolysis voltage (V)] ofat least 40,000 μF·V/g, and further preferably contains at least onekind of niobium compound selected from niobium nitride, niobium carbideand niobium boride.

In a second aspect of the invention, there is provided a process forproducing a niobium sintered body for a capacitor characterized bysintering a niobium powder containing at least one kind of niobiumcompound selected from niobium nitride,. niobium carbide and niobiumboride.

In a third aspect of the invention, there is provided a capacitorcomprising an electrode composed of the above-mentioned niobium sinteredbody of the invention, a dielectric formed on a surface of the niobiumsintered body, and a counter electrode.

BEST MODE FOR CARRYING OUT THE INVENTION

A niobium sintered body of the invention exhibiting a reduced LC valueas measured after an electrolytic oxide film is formed thereon isobtained by sintering a niobium powder wherein at least one elementselected from nitrogen, carbon and boron is bound to a part of theniobium. The amount of the bound nitrogen, carbon and/or boron, namely,the content of bound nitrogen, bound carbon and/or bound boron in theniobium powder varies depending upon the particular shape of finelydivided niobium particles, but, in the case when the niobium powder hasa particle diameter of approximately 10 μm to 30 μm, the content of eachelement is usually in the range of 50 to 200,000 ppm by weight. In viewof a reduced LC value, said content is preferably in the range ofseveral hundreds to several ten-thousands ppm by weight, and morepreferably 500 to 20,000 ppm by weight. In the case when the niobiumpowder has a particle diameter of at least approximately 3 μm, butsmaller than approximately 10 μm, said content is usually in the rangeof 50 to 50,000 ppm by weight. In view of a reduced LC value, saidcontent is preferably in the range of several hundreds to 20,000 ppm byweight, and more preferably 500 to 20,000 ppm by weight. The niobiumpowder may contain either alone or at least two of the bound nitrogen,bound carbon and bound boron, i.e., niobium nitride, niobium carbide andniobium boride.

The method of nitriding a niobium powder for forming niobium nitride maybe any of the conventional methods which include, for example, liquidnitriding, ion nitriding and gas nitriding. However, a gas nitridingcarried out in a nitrogen atmosphere is preferable because it is simpleand easy. The gas nitriding in a nitrogen atmosphere is effected byallowing a nibium powder to stand in a nitrogen atmosphere. A niobiumpowder having the objective bound nitrogen content is obtained bycarrying out the nitriding at a temperature of not higher than 2,000° C.within tens of hours. In general, the higher the nitriding temperature,the shorter the nitriding time for obtaining the desired bound nitrogencontent. Even at room temperature, when a niobium powder is allowed tostand in a nitrogen atmosphere for tens of hours, a niobium powderhaving a bound nitrogen content of approximately several tens of ppm canbe obtained.

The method of carbonizing a niobium powder for forming niobium carbidemay also be any of the conventional methods which include, for example,gas carbonization, solid carbonization and liquid carbonization. Forexample, the carbonization can be effected by allowing a niobium powderto stand together with a carbon source, for example, a carbon materialor a carbon-containing organic material such as methane at a temperatureof not higher than 2,000° C. under a reduced pressure for from severalminutes to tens of hours.

The method of boronizing a niobium powder for forming niobium boride mayalso be any of the conventional methods which include, for example, gasboronization and solid boronization. For example, niobium boride can beformed by allowing a niobium powder to stand together with a boronsource, for example, a boron pellet or a boron halide such astrifluoroboron at a temperature of not higher than 2,000° C. under areduced pressure for several minutes to tens of hours.

The niobium powder containing at least one of niobium nitride, niobiumcarbide and niobium boride preferably has an average degree of roundnessof at least 0.80. When the niobium powder having an average degree ofroundness of at least 0.80 is used, a compact having an appropriateporosity but a high packed density is obtained, and a capacitor with ananode composed of a sintered body made therefrom exhibits an enhancedwithstand voltage. Preferably the average degree of roundness is atleast 0.84.

By the term “degree of roundness” herein used, we mean a measure forexpressing the rounded shape, and the degree of roundness is defined bythe following equation.

Degree of Roundness=4π×S/L ²

wherein

S: projected area of a particle on a plane as observed when a projectionof the particle is made perpendicularly to the plane, and

L: outer peripheral length of the above-mentioned projected area.

The projected area S and outer peripheral length L of a particle can bedetermined as an expediency by taking an SEM photograph of a particle,and measuring the area of a particle image and the outer peripherallength of the area assuming that the particle image area and the outerperipheral length thereof are S and L, respectively. The average degreeof roundness is obtained by measuring the particle image area and theouter peripheral length thereof on a plurality of particle samples, forexample, at least 100 particles, and preferably at least 1,000particles. When the number of particle samples is larger, the moreprecise value is given for the determined average degree of roundness.Further, when an SEM photograph with a large magnification, for example,2,000 magnifications, is taken to obtain a particle image having a largearea, precise values for S and L can be obtained.

A niobium powder having a large average degree of roundness can beprepared, for example, by repeating a procedure of allowing a niobiumpowder, as obtained by pulverizing a niobium mass, to impinge on a flatplate, or a procedure of allowing particles of the as-obtained niobiumpowder to impinge upon another, whereby sharp edges of particles areremoved.

The niobium sintered body for a capacitor of the present invention ismade by sintering the above-described niobium powder containing at leastone of niobium nitride, niobium carbide and niobium boride. For example,the above-mentioned niobium powder is press-molded into a compact with adesired shape, and then the compact is heated at a temperature of 500 to2,000° C. under a pressure of 1 to 10⁻⁶ Torr for several minutes toseveral hours. It is to be noted that the procedure for making theniobium sintered body is by no means limited to this example.

For forming an electrolytic oxide film, the sintered body of a niobiumpowder is used as an anode in a protonic acid solution such asphosphoric acid, acetic acid, boric acid or sulfuric acid, a separatelyprepared anticorrosive metal sheet such as tantalum sheet or niobiumsheet is used as a cathode, and a voltage is imposed between the anodeand the cathode, thereby forming an electrolytic oxide film on thesurface of the niobium sintered body. The voltage imposed is usually 3to 4 times of an expected rated voltage of a capacitor with the sinteredbody of a niobium powder as an anode. In one preferable example forforming an electrolytic oxide film, a voltage is imposed between theniobium sintered body and a tantalum sheet in an aqueous phosphoric acidsolution with a 0.1% by weight concentration while the aqueous solutionwas maintained at 80° C. The time for imposing the voltage may besufficient for restoring to normal conditions defective parts occurringin the electrolytic oxide film, and a preferable time is, for example,approximately 200 minutes.

The LC value as used in the present invention is a current value asdetermined when the niobium sintered body having thereon an electrolyticoxide film is dipped in a 20% aqueous phosphoric acid solution and avoltage of 70% of the electrolysis voltage is applied for 3 minutes atroom temperature.

The LC value is not larger than 300 μA/g, and is preferably, forexample, 200 μA/g, and more preferably not larger than 200 μA/g. If theLC value exceeds 300 μA/g, the electric power consumed by the electricinstrument increases due to the defective LC and reliability thereof isreduced.

The niobium sintered body for a capacitor preferably exhibits a product(C×V) [i.e., a product of capacity (C) with electrolysis voltage (V)] ofat least 40,000 μF·V/g as measured after the electrolytic oxide film isformed thereon. When the product (C×V) is at least 40,000 μF·V/g, thespecific LC value can be desirably reduced to 5,000 [pA/(μF×V)] orsmaller. The term “specific LC value” used herein is defined as follows.In the case where an dielectric layer is formed on a surface of theniobium sintered body by electrolytic oxidation, the leak current (LC)value, as measured when a voltage of 70% of the electrolysis voltage isapplied for 3 minutes at room temperature to the niobium sintered bodyhaving a dielectric layer formed thereon, is divided by the product(C×V) to give a specific LC value. That is, the specific LC value isdefined by the following equation:

Specific LC value=LC/(C×V)

wherein LC: leak current value, C: capacity and V: electrolysis voltage.

It is presumed that the niobium sintered body of the present inventionhas the following function. Niobium has a bonding force to oxygen, whichis larger than that of tantalum, and therefore, there is a tendency ofoxygen within the electrolytic oxide film readily dispersing towardniobium metal inside the sintered body. In contrast, in the niobiumsintered body of the present invention, part of the niobium powder isbound to at least one element selected from nitrogen, carbon and boron,and therefore, oxygen within the electrolytic oxide film is difficult tobond to niobium metal and the dispersion of oxygen toward niobium metalinside the sintered body is suppressed. Nitrogen, carbon and boronstrongly bind niobium, and thus, the bonding of oxygen to the niobiumpowder having an element selected from these elements, previously boundthereto, is suppressed.

Consequently, the stability of the electrolytic oxide film is kept andLC can be reduced.

A capacitor can be manufactured from an electrode composed of theabove-mentioned niobium sintered body, a dielectric formed on a surfaceof the electrode, and a counter-electrode. The niobium sintered body ispreferably used as an anode. The manufacture of the capacitor can becarried out by an ordinary procedure. When the niobium sintered body isused as an anode, a cathode comprising at least one material selectedfrom electrolytes which are known in an aluminum electrolytic capacitorindustry, organic semiconductors and inorganic semiconductors, is usedas a cathode.

As specific examples of the organic semiconductors, there can bementioned an organic semiconductor comprising benzopyroline tetramerwith chloranil, an organic semiconductor predominantly comprisingtetrathiotetracene, an organic semiconductor predominantly comprisingtetracyano-quinodimethane, and organic semiconductors predominantlycomprising electrically conductive polymers represent by the followingformula (1) or (2), which are doped with a dopant.

In formula (1), R¹, R², R³ and R⁴ represent hydrogen, an alkyl group oran alkoxy group, R¹ and R² may form together a ring, and R³ and R⁴ mayform together a ring, X represents an oxygen, sulfur or nitrogen atom,and R⁵ exists only when X is a nitrogen atom, and R⁵ represents hydrogenor an alkyl group.

In formula (2), R¹ and R² represent hydrogen, an alkyl group or analkoxy group, R¹and R²may form together a ring, X represents an oxygen,sulfur or nitrogen atom, and R³ exists only when X is a nitrogen atom,and R³ represents hydrogen or an alkyl group.

As specific examples of the electrically conductive polymers of formula(1), there can be mentioned polyaniline, polyoxyphenylene and poly(phenylene sulfide); and, as specific examples of the electricallyconductive polymers of formula (2), there can be mentionedpolythiophene, polyfuran, polypyrrole and polymethylpyrrole.

As examples of the inorganic semiconductors, there can be mentionedinorganic semiconductors predominantly comprising lead dioxide ormanganese dioxide, and inorganic semiconductors predominantly comprisingtriiron tetraoxide.

The invention will now be described specifically by the followingworking examples.

The characteristics of a niobium powder, a niobium sintered body and acapacitor were determined by the following methods.

(1) Average Particle Diameter (μm) of Powder

Average particle diameter of a niobium powder is expressed in terms ofD₅₀ value, namely, a particle diameter (μm) when a cumulative weight in% reaches 50%, as measured by a particle size distribution measuringdevice “Microtrack”.

(2) Bound Nitrogen Content, Bound Carbon Content and Bound Boron Content

The content of bound nitrogen in a niobium powder is measured by anoxygen-nitrogen measuring device made by LECO Co. wherein nitrogencontent is determined from thermal conductivity. The content of boundboron in a niobium powder is measured by an ICP atomic emissionspectrochemical analyzer made by Shimadzu Corporation. The content ofbound carbon in a niobium powder is measured by a carbon contentmeasuring device “EMIA110” made by Horiba Mfg. Co. The measured valuesof bound nitrogen content, bound boron content and bound carbon contentare expressed as a ratio of each content to mass of the powder.

(3) Average Degree of Roundness of Powder

An SEM photograph (×2,000 magnifications) of a niobium powder is taken,the particle image is enlarged, and area (S) of the particle image andouter peripheral length (L) thereof are measured. The average degree ofroundness is calculated from the following equation.

Degree of Roundness=4πS/L ²

The degree of roundness is determined on 1,000 sample particles and theaverage value is calculated.

(4) Porosity (%) of Sintered Body

Porosity in % of a niobium sintered body is determined by a mercuryintrusion type pore distribution measuring device made by ShimadzuCorporation.

(5) Capacity (μF/g) of Sintered Body and Capacity (μF) of Capacitor

An niobium sintered body immersed in a 30% sulfuric acid and anelectrode, made of tantalum, immersed in sulfuric acid are electricallyconnected to each other via an LCR measuring device, made by HP,intervening between the niobium sintered body and the tantalumelectrode. The capacity (μF/g) of the niobium sintered body is measuredat a frequency of 120 kHz.

Capacity (μF) of a capacitor is measured by directly connecting anelectrode of the capacitor and a terminal of the LCR measuring device(Example 5).

(6) Leak Current (LC) Values (μA/g) of Sintered Body and LC Value (μA)of Capacitor

A direct current is applied between a niobium sintered body immersed ina 20% aqueous phosphoric acid solution and an electrode immersed in anaqueous phosphoric acid by imposing a voltage of 14 V, which is 70% ofthe electrolysis voltage, for 3 minutes at room temperature. Then thecurrent value (μA/g) is measured. The LC value of the niobium sinteredbody is expressed by the measured current value.

LC value (μA) of a capacitor is measured by directly connecting anelectrode of the capacitor and a terminal of the LCR measuring deviceand imposing a voltage of 10 V (Example 5).

(7) Withstand Voltage (V) of Capacitor

A voltage stepwise increasing from 1 V at an interval of 1 V is imposedin order. The voltage-imposing time is one minute in eachvoltage-imposing step. The withstand voltage (V) is expressed in termsof a voltage imposed at a step immediately before a step at which the LCvalue exceeds 50 μA.

EXAMPLE 1

(Sintered Body of Partially Nitrided Niobium Powder)

A niobium powder having an average particle diameter of 3 μm was leftstanding at 400° C. for 3 hours in a nitrogen atmosphere to give apartially nitrided niobium powder having a bound nitrogen content ofabout 3,000 ppm by weight. Then, 0.1 g of the niobium powder obtainedand a niobium lead wire were molded together to obtain a compact havinga size of 3 mm ×3.5 mm×1.8 mm. Thereafter, the compact was sintered at1,100° C. in vacuum (under a reduced pressure of 5×10⁻⁵ Torr) to obtaina niobium sintered body. Thus, 20 niobium sintered bodies were prepared.Half of the sintered bodies were electrolytically oxidized at 20 V andthe remaining were at 40 V, thereby forming an electrolytic oxide filmon the surface of each sintered body. The electrolytic oxidation wascarried out by using a tantalum plate as cathode and in an aqueous 0.1weight % phosphoric acid solution at 80° C. for 200 minutes.

EXAMPLE 2

(Sintered Body of Partially Carbonized Niobium Powder)

The same niobium powder as used in Example 1 was placed in a carboncrucible, left standing at 1,500° C. for 30 minutes under a reducedpressure, taken up into room temperature and pulverized in a vortex millto obtain a partially carbonized niobium powder having a bound carboncontent of about 1,000 ppm by weight. Subsequently, in the same manneras in Example 1, niobium sintered bodies were obtained and anelectrolytic oxide film was formed on each of the surface thereof.

EXAMPLE 3

(Sintered Body of Partially Carbonized and Partially Nitrided NiobiumPowder)

After a partially carbonized niobium powder was obtained in the samemanner as in Example 2, the partially carbonized niobium powder wasnitrided in the same manner as in Example 1 to obtain a partiallycarbonized and partially nitrided niobium powder having a bound carboncontent of about 1,000 ppm by weight and a bound nitrogen content ofabout 2,500 ppm by weight. Thereafter, in the same manner as in Example1, niobium sintered bodies were prepared and an electrolytic oxide filmwas formed on each of the surface thereof.

EXMAPLE 4

(Sintered Body of Partially Boronized Niobium Powder)

Trifluoroboron was incorporated in the same niobium powder as used inExample 1, and the mixture was left standing at 300° C. for one hourunder a reduced pressure to obtain a partially boronized niobium powderhaving a bound boron content of about 1,800 ppm by weight. Subsequently,in the same manner as in Example 1, niobium sintered bodies wereobtained and an electrolytic oxide film was formed on each of thesurface thereof.

Comparative Example 1

(Sintered Body of Untreated Niobium Powder)

Sintered niobium bodies were made and subjected to formation of anelectrolytic oxide film on each of the surface thereof in the samemanner as in Example 1 except that a niobium powder used was notnitrided.

Comparative Example 2

(Sintered Body of Tantalum Powder)

Compacts were prepared in the same manner as in Example 1 except that atantalum powder having the same particle diameter was used in place ofthe niobium powder, nitriding of powder was not carried out, and atantalum lead wire was used in place of the niobium lead wire. Theresulting compacts were sintered at 1,700° C. in vacuum to obtaintantalum sintered bodies. Thereafter, an electrolytic oxide film wasformed on each of the surface thereof in the same manner as in Example1.

Evaluation of Sintered Body Having Electrolytic Oxide Film FormedThereon

Average capacity per unit weight and average LC value of the sinteredbodies having formed thereon an electrolytic oxide film were determined.The results are shown in Table 1. (C ×V) value was calculated fromelectrolysis voltage (V) and capacity (C), and specific LC value wascalculated from LC and (C×V). The results are also shown in Table 1.

TABLE 1 Elec- trolysis Capacity Specific voltage (a) (b) LC CV (a × b)LC value (V) (μF/g) (μA/g) (μF · V/g) [pA/(μF × V)] Ex. 1 20 2,000 15040,000 3,750 40 1,000 200 40,000 5,000 Ex. 2 20 1,800 180 36,000 5,00040 900 210 36,000 5,833 Ex. 3 20 2,000 150 40,000 3,750 40 1,000 18040,000 4,500 Ex. 4 20 2,000 160 40,000 4,000 40 1,000 190 40,000 4,750Co.Ex.1 20 1,500 800 30,000 26,666 40 700 900 28,000 32,143 Co.Ex.2 20500 5 10,000 500 40 250 6 10,000 600

As seen from Table 1, when a niobium sintered body of the presentinvention, containing at least one of niobium nitride, nibioum carbideand niobium boride is electrolytically oxidized to form an electrolyticoxide film thereon, the resulting sintered body has an LC value notlarger than 300 μA/g, more preferabaly not larger than 200 μA/g. When CVof the sintered body is enhanced to 40,000 μF·V/g or more, a capacitorhaving a specific value of not larger than 5,000 μpA/(μF×V) can beobtained.

EXAMPLE 5

In this example, effects of the degree of roundness of a nobium powderon the porosity and packed density of a niobium sintered body, and thewithstand voltage and LC value of a capacitor were examined.

A commercially available niobium powder having an average degree ofroundness of 0.72 and an average particle diameter of 40 μm round by ajet mill (specimen No. 1 to 8) or a vibrating mill (specimen No. 9 to12) whereby finely divided niobium particles were impinged against eachother to give a niobium powder having an average degree of roundnessshown in Table 2. By varying the residence time in each mill, niobiumpowders having different average degrees of roundness were prepared. Theniobium powders were classified into an average particle diameter in therange of 7 to 8 μm.

Each niobium powder was allowed to stand at 600° C. for 3 hours in anitrogen atmosphere to prepare a partially nitrided niobium powderhaving a bound nitrogen content of approximately 3,000 ppm. When thenitriding was conducted, the degree of roundness of the niobium powderdid not vary.

The partially nitrided niobium powder was press-formed into a compacthaving a diameter of 10 mm and a thickness of about 1 mm. The compactwas sintered at 1,500° C. under a pressure of 10⁻⁵ Torr for 30 minutesto obtain a niobium sintered body having a weight of 0.30 g. By varyingthe pressure applied upon press-forming into the compact, variousniobium sintered bodies having different porosities were prepared. Amongthese, sintered bodies having a porosity of 53% or 45% were fabricatedinto capacitors as follows.

Each niobium sintered body was electrolytically oxidized at 65 V in anaqueous phosphoric acid solution to form a niobium oxide dielectric filmon the surface of the sintered body. The electrolytically oxidizedsintered body was immersed in an aqueous manganese nitrate solution, andthen, the sintered body was taken out from the solution and subjected toa decomposing treatment at 250° C. This procedure of immersing in anaqueous manganese nitrate solution and then decomposing at 250° C. wasrepeated to form a manganese dioxide dielectric semiconductor layer onthe niobium oxide dielectric film. Further, a carbon paste and then asilver paste were coated in this order on the semiconductor layer, andfinally the coated sintered body was encapsulated with an epoxy resin toobtain a capacitor.

The average degree of roundness of the niobium powder, the prosity andpacked density of the sintered body, and the capacity, withstand voltageand LC value at 10 V were evaluated. The results are shown in Table 2.

TABLE 2 Capacitor Nb powder Sintered body With- Average Packed stand LCSpecimen degree of Porosity density Capacity voltage value No. roundness(%) (g/cm²) (μF) (V) (μA) 1 0.93 53 4.9 101 78 0.4 2 0.89 53 4.8 101 760.5 3 0.84 53 4.7 100 75 0.3 4 0.80 53 4.6 97 73 0.6 5 0.93 45 5.4 11176 0.7 6 0.89 45 5.2 108 76 0.4 7 0.84 45 5.0 104 76 0.3 8 0.80 45 4.8101 73 0.5 9 0.77 53 4.4 86 69 0.6 10 0.75 53 4.3 85 68 0.7 11 0.77 454.4 89 69 0.7 12 0.75 45 4.4 88 68 0.5

INDUSTRIAL APPLICABILITY

A capacitor manufactured from the niobium powder sintered of the presentinvention has a large capacity per unit weight, reduced leak current(LC) value as compared with a LC value he conventional capacitor. Thus,the capacitor made from sintered body of the present invention has alarge capacity although its size is small.

Especially when the sintered body of the present invention is made of aniobium powder having a large average degree of roundness, it has arelatively large porosity and a good packed density, and a capacitormanufactured from this sintered body has a large capacity and goodwithstand voltage characteristics.

What is claimed is:
 1. A niobium sintered body for a capacitor, whichexhibits an LC value of not larger than 300 μA/g as measured after anelectrolytic oxide film is formed thereon, and is made of a niobiumpowder having a degree of roundness of at least 0.8 as defined by thefollowing equation: Degree of roundness=4π×S/L ² wherein S: projectedarea of a particle on a plane as observed when a projection of theparticle is made perpendicularly to the plane, and L: outer peripherallength of the above-mentioned projected area.
 2. A capacitor having anelectrode comprising a niobium sintered body, a dielectric formed on asurface of the niobium sintered body, and a counter electrode; saidniobium sintered body being made of a niobium powder and exhibiting anLC value of not larger than 300 μA/g as measured after the dielectric isformed on a surface of the niobium sintered body, wherein the niobiumsintered body is made of a niobium powder having a degree of roundnessof at least 0.8 as defined by the following equation: Degree ofroundness=4π×S/L ² wherein S: projected area of a particle on a plane asobserved when a projection of the particle is made perpendicularly tothe plane, and L: outer peripheral length of the above-mentionedprojected area.
 3. A process for producing a niobium sintered body for acapacitor comprising the step of sintering a niobium powder containingat least one kind of niobium compound selected from the group consistingof niobium nitride, niobium carbide and niobium boride.
 4. The processfor producing a niobium sintered body for a capacitor according to claim3, wherein the niobium sintered body contains bound nitrogen, boundcarbon or bound boron in an amount in the range of 50 ppm by weight to200,000 ppm by weight.
 5. The process for producing a niobium sinteredbody for a capacitor according to claim 3, wherein the niobium powderhas an average particle diameter of 3 μm to 30 μm.
 6. The process forproducing a niobium sintered body for a capacitor according to claim 3,wherein the niobium powder has a degree of roundness of at least 0.8 asdefined by the following equation: Degree of roundness=4π×S/L ² whereinS: projected area of a particle on a plane as observed when a projectionof the particle is made perpendicularly to the plane, and L: outerperipheral length of the above-mentioned projection area.
 7. The processfor producing a niobium sintered body for a capacitor according to claim3, wherein the niobium sintered body produced exhibits an LC value ofnot larger than 300 μA/g as measured after an electrolytic oxide film isformed thereon.
 8. The process for producing a niobium sintered body fora capacitor according to claim 3, wherein the niobium sintered bodyproduced has a product (CV), which is a product of capacity (C) withelectrolysis voltage (V), of at least 40,000 μF·V per g.
 9. The niobiumsintered body for a capacitor according to claim 1, which has a product(CV), which is a product of capacity (C) with electrolysis voltage, (V),of at least 40,000 μF·V per g.
 10. The niobium sintered body for acapacitor according to claim 1, which is made of a niobium powdercontaining at least one compound selected from the group consisting ofniobium nitride, niobium carbide and niobium boride.
 11. The niobiumsintered body for a capacitor according to claim 1, wherein the niobiumsintered body contains bound nitrogen, bound carbon or bound boron in anamount in the range of 50 ppm by weight to 200,000 ppm by weight. 12.The niobium sintered body for a capacitor according to claim 1, whereinthe niobium sintered body is made of a niobium powder having an averageparticle diameter of 3 μm to 30 μm.
 13. The capacitor according to claim2, wherein the dielectric formed on a surface of the niobium sinteredbody comprises niobium oxide formed by electrolytic oxidation.
 14. Thecapacitor according to claim 2, which has a product (CV), which is aproduct of capacity (C) with electrolysis voltage (V), of at least40,000 μF·V per g.
 15. The capacitor according to claim 2, which is madeof a niobium powder containing at least one compound selected from thegroup consisting of niobium nitride, niobium carbide and niobium boride.16. The capacitor according to claim 2, wherein the niobium sinteredbody contains bound nitrogen, bound carbon or bound boron in an amountin the range of 50 ppm by weight to 200,000 ppm by weight.
 17. Thecapacitor according to claim 2, wherein the niobium sintered body ismade of a niobium powder having an average particle diameter of 3 μm to30 μm.